Accounts of Chemical Research, Volume 42, Issue 12, Page 1974-1982, December 21, 2009. [...]
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Theoretical investigations of ionic liquids are reviewed. Three main cate-gories are discussed, i.e., static quantum chemical
calculations (electronic structure methods), traditional molecular dynamics simulations and first-principles molecular dynamics simulations. Simple models are reviewed in brief.
Task specific ionic liquids (TSILs), or more generally task specific onium salts (TSOSs), can be defined as an association
of a cation and anion, one at least being organic, to which has covalently been attached through a linker a function that confers the assembly a specific task. After presentation of the general concept of TSILs and TSOSs, the different methods of preparation of these compounds are developed. Regarding their applications in chemistry, TSILs and TSOSs can be used as soluble supports for reagents and catalysts in multiphasic reactions, enabling high activity and easy recovery of the supported agent. However, additionally, they can be used as soluble supports for organic synthesis in a similar manner to resins and offer several advantages over traditional methods.
Ionic liquids have shown to be excellent solvents for optical investigation of solutes. Transition metal complexes as well
as f-element compounds have been studied in ionic liquids. Not only are neat and clean ionic liquids transparent in the NIR and visible region, but the absence of anions with low frequency oscillators such as C–H, N–H and O–H have been found to be favourable when it comes to photoluminescence and future applications can be envisioned. Of fundamental interest is the study of the absorption spectra of organic dyes dissolved in an ionic liquid as the many physicochemical parameters such as the dipolarity and polarizability of the ionic liquid and its hydrogen bond ability can be determined.
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The mechanism of chemical reactions between electron donors and acceptors continuously changes with the power of the donors
and the acceptors. The interaction between the HOMO (d) of the donors and the LUMO (a*) of the acceptors or delocalization of electrons is important for the reactions. The electron d-to-a* transferred configuration mixes to a significant extent. As the donors and/or the acceptors are strong, their excited configurations appreciably mixes together with the transferred configuration. The d–a and d*–a* orbital interactions are important in addition to the d–a* interaction. Reactions have features characteristic of the excited-state reactions although the donor–acceptor system is not really excited, but in the ground state. This process is termed pseudoexcitation. The a–d–a*–d* interaction is important. For much stronger donors and acceptors, the electron transferred configuration is stable and predominant. Covalent bonds do not form but instead ionic pairs, and electron transfer results. A mechanistic spectrum of chemical reactions is composed of the delocalization, pseudoexcitation, and electron transfer bands.
Today, NMR spectroscopy is the most important analytical tool for synthetically working chemists. This review describes the development of NMR spectroscopic
methods for use in ionic liquid media and the state-of-the art in terms of routine analytics as well as modern advanced techniques.
Understanding the ways in which the constituents of ionic liquids, i.e. the type of cation, its substitution, and the type
of anion chosen, interact with reactants is prerequisite to deliberately designing an ionic liquid solvent with optimum performance. Several approaches, including physico-chemical and spectroscopic measurements and computational studies of binary ionic liquid-substrate mixtures have been presented that investigate the strength of interactions. The qualitative order of the basicity (hydrogen bond acceptor potential) of anions as most prominent force is already reasonably
well understood, and reliably determined using, e.g. selective solvatochromic dyes. In certain reactions, the relative order of basicity correlates well with the reactivity of substrates. However, the determination of a relative order for the cations is still in its infancy. Owing to the fact that potential cation-derived interactions may not solely be due to hydrogen bond interactions, but also to ion pair interactions (electron pair donor/acceptor properties), the relative magnitudes of interactions between the anion and cation vary considerably – even in the absence of solutes – depending on the experimental method. In addition, it appears that the basicity of the anion superimposes in many instances on the effects exhibited by the cation and/or the cation’s substituent. Hence, understanding the effect of the cation on the activation of substrates is still a challenge. This chapter aims at summarising the trends observed for binary model systems in experimental and computational investigations,
and drawing conclusions about ionic liquid structure-induced effects relevant to organic reactions, in particular nucleophilic substitution reactions.
A theory of the interaction of three orbitals, i.e., the φ
h –φ p –φ l interaction, and its chemical applications are reviewed. General rules are drawn to predict the orbital phase relations between φ h and φ l, which do not directly interact with each other but indirectly through a perturbing orbital φ p. When φ h and φ l are orbitals on the same atoms, bonds, or molecules and φ p is a perturbing orbital, φ h deforms by mixing-in of φ l and vice versa. The direction of the orbital deformation is determined by the orbital phase relation between φ h and φ l. The orbital mixing rules are applied to the deformation of the orbitals. The deformation determines favorable interactions with other orbitals. The orbital mixing rule is powerful for understanding and designing selective reactions. The electrostatic orbital mixing by positive and negative electric charges and its chemical consequences are reviewed as well.
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